(1) Field of the Invention
The present invention relates to a mobile communication system, and a radio station and a mobile terminal. Particularly, the present invention relates to a technique suitable for use in mobile communications in which communication is carried out in code division multiple access (CDMA).
(2) Description of Related Art
CDMA is widely used as a mobile communication system. Frequency division multiple access (FDMA) or time division multiplex access (TDMA) is basically a system operated under conditions without interference between subscribers since it assigns resources (frequencies, time or the like) orthogonal to one another to subscribers (mobile terminals such as portable telephones or the like). On the other hand, CDMA is operated under conditions that subscribers are interfered with each other, thus being a system the channel capacity of which is expected to be improved by the statistical multiplexing effect.
Since the CDMA communication system is operated under conditions that signals of subscribers are interfered with each other as above, transmission power control (TPC) or forward error correction (FEC) or the like is used to control a transmitting signal power of each subscriber to be the minimum requirement, minimize the interference between them, thereby to maximize the channel capacity.
It is said that the channel capacity of a CDMA communication system is generally calculated by the following equation (1) (reference 1; A. Viterbi, CDMA Principles of Speed Spectrum Communication Addison-Wesley, (1995)):
                    N        ≈                              pg                                          Eb                /                No_th                            ×                              (                                  fs                  +                  fo                                )                            ×              d                                ×          Dt          ×                      Gs            Ns                    ×          Lf                                    (        1        )            
In the above equation (1), N is a channel capacity (ch/sector/RF), pg is a processing gain, Eb/No_th is a required Eb/No, fs is a self-cell interference ratio, fo is an other-cell interference ratio, d is a voice activity factor, Dt is degradation due to a transmitting power control error, Gs is a sectorization effect, Ns is the number of sectors/cells, and Lf is a loading factor. Incidentally, the channel capacity is considered using the following equation (2), omitting parameters not relating to the discussion:
                    N        ≈                  K          ×                      pg                                          Eb                /                No_th                            ×                              (                                  fs                  +                  fo                                )                                                                        (        2        )            
In the equation (2), K represents a factor of an effect of other parameter.
The equation (2) signifies that when a signal-to-(interference+noise)power ratio (required Eb/No) required to satisfy desired communication quality (hereinafter also referred as transmission quality) increases, a transmission power of each station increases, thus interference is increases, which leads to a decrease in channel capacity N.
CDMA is a system that optimizes the communication quality and the channel capacity by TPC and FEC. However, CDMA has a disadvantage that the required Eb/No is degraded at a specific terminal moving speed because of a difference in response speed between TPC and FEC (reference 2; R. Padovani, Reverse Link Performance of IS-95 Based Cellular Systems IEEE Personal Communications, Third Quarter, (1994)).
This phenomenon occurs as follows:
In the mobile communication, there occurs fading (Reyleigh fading) that propagation loss fluctuates due to mainly interference between reflections from objects around the terminal, which fluctuates at a speed according to a moving speed of the terminal. The fluctuating speed of Reyleigh fading is characterized by a Doppler frequency fD due to movement of the terminal as shown by the following equation (3) (reference 3; Okumura, Shinshi, “Fundamentals of Mobile Communications”, The Institute of Electronics Information and Communication Engineers, (1986)):fD≈fR×VM/C  (3)where fR is a radio frequency, VM is a terminal moving speed and C is the speed of light.
It is seen that higher the radio frequency fR and faster the terminal moving speed VM, faster is the fluctuating speed of Reyleigh fading. Since the response speed is generally slow in TPC, it is possible to follow the propagation loss fluctuation due to fading to control the transmitting power when the Reyleigh fading fluctuation is slow (that is, when the Doppler frequency fD is low), thus degradation of the communication quality can be prevented. However, when the Doppler frequency (that is, the fading frequency) fD is high, the communication quality is degraded since TPC cannot follow the fading fluctuation.
On the other hand, it is possible to disperse an effect of burst signal power reduction due to fading by combining FEC with interleaving (IL). However, when the fading speed is slow, there occurs signal power reduction for such a long time that the signal power reduction cannot be corrected since the IL cycle is finite, which leads to degradation of the communication quality.
For this, there occurs a phenomenon that noticeable reduction of the communication quality at the intermediate frequency fD between the Doppler frequency fD at which an effect of improvement of the communication quantity by TPC is obtained and the Doppler frequency fD at which an effect of improvement by FEC is obtained (refer to FIG. 13 and the reference 2). FIG. 13 shows Eb/No required to satisfy the frame error rate (FER)=1% in a reverse link (mobile terminal to base station, 9.6 kbps) in an IS-95 system. When the radio frequency=850 MHz, fD=40 Hz corresponds to a mobile terminal moving speed=50.8 km/h, for example.
Since the terminal moving speed cannot be defined uniquely, the system has to be designed at the Doppler frequency at which the quality degradation is maximum [namely, the required Eb/No is maximum (in the example in FIG. 13, fD=47 Hz and the required Eb/No=6.1 dB)]. However, a large required Eb/No degrades the channel capacity N as expressed by the above equation (2). On the other hand, when the channel capacity N is secured, the required Eb/No cannot be satisfied, which leads to degradation of the communication quality.
In a system such as TDMA or FDMA in which the transmission quality monotonically degrades when the fading pitch (fD) increases, an attempt is made to prevent large degradation of the transmission quality of a specific terminal to equalize the communication quality in the entire system, thereby improving the overall performance, as disclosed in, for example, Japanese Patent Laid-Open Publication No. 5-259969.
In concrete, the technique disclosed in Japanese Patent Laid-Open Publication No. 5-259969 (hereinafter referred as known technique 1) accomplishes the above effect by a simple control based on only magnitude of the terminal moving speed on the assumption of monotonousness of the Doppler frequency fD and the transmission quality degradation that degradation of the transmission quality decreases when the fading pitch becomes smaller. However, such a simple control can provide only a small effect when degradation of the transmission quality to the Doppler frequency is non-monotonous as in CDMA (when there is employed a communication system having a characteristic that a required signal-to-noise power ratio of a received signal in a mobile terminal changes from a tendency to increase to a tendency to decrease according to the moving speed of the mobile terminal) which sometimes causes degradation of the communication quality in some cases.
Namely, the known technique 1 tries to assure the communication quality of a control channel by assigning a low frequency band to the control channel such that fD (∝(moving speed)×(radio frequency)) of the control channel required a high communication quality becomes as low as possible. However, when the Doppler frequency fD is lowered in a system such as CDMA in which degradation of the transmission quality to the Doppler frequency fD is non-monotonous in the similar manner, there is a case where the communication quality degrades as with the case where FD>42 Hz in FIG. 13, for example.
The known technique 1 accommodates traffic of a high-speed mobile terminal in a low radio frequency band and a low-speed moving terminal in a high radio frequency band to equalize the Doppler frequency fD (∝(moving speed)×(radio frequency)) as much as possible, thereby homogenizing the transmission quality to improve the overall performance.
In this method, when a radio frequency band in which the Doppler frequency brings about the worst value of the transmission quality (fD=42 Hz in the example in FIG. 13) is assigned, the transmission quality of the both terminals becomes the worst, thus the overall performance becomes the worst. The known technique 1 cannot prevent this.
The above problem is caused by that the known technique 1 performs a simple control based on only magnitude of the terminal moving speed on the assumption of monotonousness of the Doppler frequency fD and the transmission quality degradation.
In consideration of assignment of frequencies in a CDMA communication system, there is a technique disclosed in, for example, Japanese Patent Laid-Open Publication No. 10-23502 (hereinafter referred as known technique 2) as a known technique. An objective of the known technique 2 is to equalize the number of terminals to be accommodated in each radio frequency band (strictly, the number of radio channels to be used) in order to secure a channel for soft hand-off in the CDMA communication system.
However, the known technique 2 does not consider non-monotonousness or the like of the transmission quality to the terminal moving speed and the Doppler frequency fD, but assigns a radio frequency band without considering the terminal moving speed. For this, when the Doppler frequency fD brings about the worst value of the transmission quality at the assigned frequency (fD=42 Hz in the example in FIG. 13), large degradation occurs.